A turbine of this type comprises stages, each of which comprises a turbine wheel and a stator nozzle, each nozzle being sectorized, i.e. being made up of a plurality of nozzle sectors that are disposed circumferentially end to end.
Each nozzle sector has two annular platform sectors extending one inside the other and interconnected by substantially radial vanes. The outer platform has means for fastening to an outer casing of the turbine. The nozzle sector includes an annular rail sector for supporting elements made of abradable material, the rail being situated radially inside the inner platform of the nozzle and being connected to the inside surface of said platform. The abradable material elements co-operate with annular wipers carried by the rotor of the turbine so as to form labyrinth type seals.
The nozzle sectors are separated from one another by small clearances in the circumferential direction so as to accommodate thermal expansions of their platforms when the turbine is in operation.
In operation they are subjected to vibration and to dynamic stresses that are relatively large and that can lead to small parasitic movements of the nozzle sectors and to deformations of these sectors, in particular in twisting.
Proposals have already been made to stiffen the nozzle with the help of axial bearing means formed on the sectors of the inner platform of the nozzle, the bearing means of one platform sector being designed to co-operate with corresponding means formed on the adjacent inner platform sectors in order to limit deformation of the nozzle in operation.
In the prior art, each inner platform sector has longitudinally-extending edges that are cut to be substantially Z-shaped and that are complementary to the corresponding longitudinally-extending edges of adjacent inner platform sectors. Each Z-shaped longitudinally-extending edge of a platform sector comprises two end portions parallel to the longitudinal direction that are offset in the circumferential direction and that are connected to each other by a perpendicular margin for coming to bear axially against the corresponding margin of an adjacent platform sector while the turbine is in operation so as to limit the above-mentioned parasitic movements and deformations of the nozzle.
However, that technology presents drawbacks. The longitudinally-extending edges of each platform sector need to be machined so as to from the Z-shaped cutouts. This machining is a difficult operation that runs the risk of damaging the nozzle. The machining of these longitudinally-extending edges consists in particular in making a first cut to form a bearing margin and a second cut to connect said bearing margin to an upstream or downstream circumferential edge of the inner platform. These cuts are made close to the vanes of the nozzle, and the curved shape of the vanes can interfere to a greater or lesser extent with the machining operation. Finally, that technology is applicable only to nozzle inner platforms that are relatively plane and cannot be generalized to all types of nozzle or of nozzle sector.